A filter transmitting a normal mode signal and absorbing common mode noise has been known (Japanese Unexamined Patent Application Publication Nos. 8-115820, 9-214276, and 2009-135760).
In the filter disclosed in Japanese Unexamined Patent Application Publication No. 8-115820, an input terminal and an output terminal of a first choke coil are connected to an input terminal and an output terminal of the filter, respectively. The first choke coil transmits a normal mode signal and blocks common mode noise. Furthermore, an input terminal of a second choke coil is connected to the input terminal of the filter. The second choke coil blocks the normal mode signal and transmits the common mode noise. Each of two output terminals of the second choke coil is grounded with a terminal resistor interposed therebetween. The terminal resistors convert energy of the common mode noise into heat for consumption.
In the filter disclosed in Japanese Unexamined Patent Application Publication No. 9-214276, a series circuit of a capacitor and a resistor is connected between input terminals thereof. In the filter, the resistor absorbs energy of a noise component that the filter does not transmit and converts it into heat energy for consumption.
The filter disclosed in Japanese Unexamined Patent Application Publication No. 2009-135760 is arranged in a shield case shielding an electronic apparatus main body including a signal generator. The electronic apparatus main body has ports for extracting, to the outside, a signal while passing through the shield case, and a common mode choke coil for radiation noise prevention, which is connected to a transmission line connecting the signal generator and the ports. The common mode choke coil is arranged in a substrate or a connector located at a position distanced from shield surfaces of the respective ports by less than about ¼ of a wavelength of a signal frequency at the inner side or the outer side of the shield surfaces, and a common mode impedance of the common mode choke coil is set to be equal to or larger than about 377Ω. This configuration reduces radiation noise to the outside of the shield case.
Japanese Unexamined Patent Application Publication Nos. 8-115820 and 9-214276 disclose a filter circuit in which the resistor converts energy of the noise component into heat for consumption but do not disclose a specific mounting position of the filter circuit and do not refer to an effective usage method thereof.
The electronic apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2009-135760 can reduce the radiation noise by optimizing an insertion position of the common mode choke coil. The electronic apparatus cannot however provide a sufficient effect against a phenomenon called self-poisoning electromagnetic noise generated in the electronic apparatus that adversely influences other functions in the electronic apparatus.